This invention relates to a cladding glass for an optical fiber, more particularly to a cladding glass for an optical fiber having low refractive index and low viscosity.
An optical glass fiber is generally constituted of a core glass and a cladding glass which covers around the core glass. Such a cladding glass is required to have a refractive index smaller than that of the core glass. Thus, the light incident upon the core glass is confined within the core glass while repeating total reflections at the boundary surface between the core glass and the cladding glass to be transmitted toward the other end.
In such an optical fiber, the cladding glass should preferably have a low refractive index. This is because the numerical apperture of the fiber is increased by making the difference in refractive index between the core glass and the cladding glass, whereby the ability to confine the light to be transmitted within the core glass can be enhanced.
In order to make greater the refractive index difference as mentioned above, one may in the first place think of increasing the refractive index of the core glass. However, in that case, the core glass becomes readily crystallizable and tendency to deteriorate weathering resistance and increase scattering loss. Thus, increase of the refractive index of the core glass is limited of itself.
Another method is to lower the refractive index of the cladding glass. For this purpose, components such as an alkali metal, an alkaline earth metal, etc. in the glass may be reduced. However, the following inconvenience is caused. That is, due to elevation of melting temperature, no sufficient vitrification can proceed at a practical temperature (e.g. 1400 .degree. C. or lower), whereby it is rendered factually impossible to obtain a homogeneous glass. Also, such a glass becomes highly viscous to exhibit a great viscosity difference from the core glass during fiber drawing. As the result, during fiber drawing according to the double crucible method conventionally practiced, it becomes difficult to adjust the ratio of core diameter to cladding diameter, and the fiber obtained is lowered in dimensional uniformity.
For reducing the viscosity of a highly viscous glass, the amount of the components which have low melting temperature such as an alkali metal, boron oxide, etc. may be increased. However, in that case, the glass obtained is increased in refractive index and also the problems such as deterioration in water resistance, crystallization, etc. will occur.
Thus, it would be very desirable to develop a glass endowed with both properties of low refractive index and lowered viscosity during drawing as the cladding glass for an optical fiber.